In modern physics, small-type refrigerating machines are increasingly being used to generate low temperatures (70.degree. K. to 20.degree. K.) with refrigerating capacities of up to 5 W. One of the most important functional elements of such small-type refrigerating machines is a regenerative heat exchanger (regenerator).
In the first stage, in the temperature range from 300.degree. K. to 70.degree. K., the corresponding high-capacity regenerators of Gifford-McMahon use a regenerator packing of wire meshing with wire diameters from 0.05 to 0.03 mm. For the second temperature stage, with working-gas temperatures below 70.degree. K., oxide-free lead powder is used on account of its high volumetric heat capacity, since no lead wire meshing with the required dimensions is available (Walker, Cryocoolers, Part II, p. 45, Plenum Press, 1983).
The desired optimum porosity of the low-temperature packing is at 0.05 to 0.1 (Radebaugh, First Step to the Optimization of Regenerator Geometry, NBS-SP-698, May 1985). This desired optimum arises from the partly oppositely acting individual loss mechanisms of the heat transfer, of the limited specific heat capacity, of the flow pressure losses, of the dead volume and of the axial heat conduction. The prior art can achieve the aimed for porosity optimum of 0.05 to 0.1 by means of lead powder filling; practically, however, porosities of only 0.37 to 0.4 are achieved. With ideal dense spherical packings, a value of 0.25 could be attained. However, the lead powder particles used, with average diameters of 0.1 to 0.25 mm, cannot be produced with an ideal spherical shape. Furthermore, the operating conditions of a small-type refrigerating machine require that the working gas flows rapidly around the matrix body. When conventional spherical bed fillings are used, a semi-fluid state of the fixed bed results from the flow pressure loss at the individual particles, since the pressure loss is of the order of the particle weight per flow surface. This causes a swirling of the particles. For this reason, the spherical bed filling must be subjected to mechanical pressure. The mechanism required for this increases the dead volume in the regenerator. This has a negative effect on the conduct of the process.
German Offenlegungsschrift 3,044,427 discloses a sintered metal for use in the low temperature section of the regenerator. Low porosities can indeed be achieved with this material. However, these sintered metals have heat bridges that conduct well. This causes undesirably high heat conduction and thus once again leads to large losses of effectiveness.